1. Field of the Invention
The present invention relates to boat propelling apparatuses and boats. More specifically, the present invention relates to a boat propelling apparatus including an engine which has a plurality of cylinders, and to a boat equipped therewith.
2. Description of the Related Art
In general, boat propelling apparatuses equipped in boats have different engine output characteristics depending upon the purpose for which the boat serves. For example, boats for cruising purposes are expected to make a high-speed, long-time travel, and in this type of boat, the engine output characteristics of the boat propelling apparatus are adjusted so that high torques will be obtained in the high speed range. Likewise, bass fishing boats for example, are expected to have a superior mobility in a middle to slow speed range, and in this type of boat, the engine output characteristics of the boat propelling apparatus are adjusted so that high torques will be obtained in the middle to slow speed range.
The engine output characteristics can be adjusted by changing the length of air intake passage (air intake pipe) thereby making adjustment on an intake-air pulsation effect. Specifically, the engine output torques in a high speed range can be increased by decreasing the length of the air intake passage, and the engine output torque in a middle to slow speed range can be increased by increasing the length of the air intake passage. Changing the length of the air intake pipe based on this relationship thereby adjusting the engine output characteristic to different settings without changing the other design specifications of the engine is a conventional practice.
However, it is difficult to achieve a large increase in the length of the air intake passage (air intake pipe) in cases where there is not sufficient space available for the engine due to the design limitation. As a solution to this problem, JP-A Hei 7-102978 discloses an air supply apparatus for a V-type engine, where an air supply pipe is communicated with a collector and this collector is placed in a surge tank. The arrangement makes it possible to increase the length of the air supply pipe within a small engine room. Also in this air supply apparatus, the collector is provided with an adjusting means such as a damper, and the engine output characteristic is also adjusted by adjusting the amount of air flow.
As another example, JP-A 2004-162710 discloses an intake system for internal-combustion engine, where an intake pipe is divided into an upper intake part which is connected with an intake plenum, and a lower intake part which is fixed to the cylinder head of the internal combustion engine. Part of the upper intake part is inside the intake plenum. In this intake system, the upper intake part is replaceable with another to change the length of the portion inside the intake plenum, whereby the total length of the intake pipe can be changed. This arrangement makes it possible to change the length of the intake pipe without much changing the geometric shape of the intake system itself.
However, through validation processes from various aspects, the inventors of the present invention have found that it is difficult to make an adjustment on the engine output characteristic according to the air intake apparatus disclosed in JP-A Hei 7-102978. Specifically, in the air supply apparatus according to JP-A Hei 7-102978, it is impossible to change the length of individual air intake passages (air intake pipes) which are provided downstream from the surge tank. Due to this limitation, it is impossible to make sufficient adjustment on the intake-air pulsation effect, and therefore it is impossible to adjust the engine output characteristic sufficiently. Also, in cases where the adjusting means such as a damper is to be provided for adjustment on the amount of air flow, the air supply apparatus has to have a complicated structure, which leads to a problem of increased cost of manufacture.
There is a cost problem also in the intake system according to JP-A 2004-162710. Specifically, the invention described in JP-A 2004-162710 requires at least two kinds of upper intake parts in order to adjust the engine output characteristic. This increases manufacturing cost. The intake system according to JP-A 2004-162710 also requires an increased level of manufacturing expertise since the upper intake parts have different shapes from each other but they have to be assembled to a common lower intake part.